In recent years, various proposals have been made to a distributed system in which various kinds of devices on a field (hereinafter referred to as “field devices”) are interconnected through a network (for example, see Patent Literature 1 to 4). FIG. 15A shows a configuration of a conventional distributed system 200. The distributed system 200 is a system in which field devices 70 and 80 are connected to each other through a network N. The network N is a network such as a public line, a telephone line, an ISDN line, a wireless communication line, an exclusive line, a CATV line, an optical communication line. The number of field devices constituting the distributed system 200 is not limited to a special value.
Examples of the distributed system as described above include a PA (Process Automation) system, an FA (Factory Automation) system, a broadband monitoring/control system, a measuring system, an industrial/social system. Examples of the field devices constituting the distributed system include a network device such as a sensor, an actuator, a controller, a communication measuring instrument, a measurement instrument, an IC tester, a camera, a router, a switch or the like.
A processing flow when the distributed system 200 runs a plurality of distributed applications in parallel will be described with reference to FIG. 15B. A distributed application is an application implemented in the distributed system by allocating functions constituting the distributed application to field devices as tasks and executing the tasks in cooperation with one another. The distributed application will be hereinafter referred to as “distributed AP”.
When running the distributed AP, it is necessary to assign task priorities of the field devices to the tasks constituting the distributed AP. A task priority is an OS-specific index which is used to allocate a CPU (Central Processing Unit) of the field device to a task by an OS (Operating System) and which represents order of priority of the task processing. A task having a higher task priority is executed prior to a task having a lower task priority. Tasks having the same priority are equally time-divisionally processed. Here, the task is equivalent to a process or a thread, and these will be representatively called as “task”.
In the distributed system 200, one or more field devices in which tasks are executed are first determined. For example, as shown in FIG. 15B, when each of two distributed APs (a distributed AP1 and a distributed AP2) has three tasks, the field device 70 is determined as a field device which executes two tasks of the distributed AP1 and one task of the distributed AP2, and the field device 80 is determined as a field device which executes the remaining one task of the distributed AP1 and the remaining two tasks of the distributed AP2. When one or more field devices for executing tasks are determined, one or more hardware resources necessary for the tasks are allocated by using OS functions of the field devices. With respect to the order of priority of the processing, a task priority specific to an OS is designated. When a plurality of tasks are executed by a single field device, the order of priority among the tasks is determined, and the task priorities specific to an OS are allocated.    Patent Literature 1: Japanese Patent Application Laid-Open No. 10-215492    Patent Literature 2: Japanese Patent Application Laid-Open No. 2001-51716    Patent Literature 3: Japanese Patent Application Laid-Open No. 2002-318702    Patent Literature 4: Japanese Patent Application Laid-Open No. 2004-54803